The government has tried in many ways, including regulations, to limit or even prevent the pollution of surface water. For example, environmental permits generally stipulate the volume of waste products that a company may release into the surface water in a given period.

Air pollution

Discharges to the air are also governed by regulations, because
air pollution can lead via precipitation to water pollution.
Mercury, for example, now enters the environment chiefly as a
result of the burning of fossil fuels. In the past mercury
compounds were often used in pesticides and in the production of
chlorine. In moist environments micro-organisms transform inorganic
mercury into organic mercury, which is more poisonous. Organic
mercury compounds, such as methyl mercury, can accumulate in the
food chain and are dangerous for the central nervous system.

Very persistent pesticides

In the 1950s and 1960s very persistent (poorly degradable)
pesticides such as lindane and DDT were widely used. A chemical
such as DDT can spread through water over the whole world. DDT
itself has been found in the ice of Antarctica. Other poorly
degradable compounds such as PCBs are also still found in the
environment, although its use was restricted long ago. PCBs were
used in electrical equipment and as lubricants, among other
applications.

One of the characteristics of these compounds is that they can
accumulate in the food chain. This means that the levels in higher
organisms, including fish, molluscs and crustacea, are many times
higher than the concentrations in water.

Directive 93/351/EEC

In Directive 93/351/EEC the European Commission has set the
maximum levels of mercury that may be found in samples of halibut,
tuna, wolf fish and eels at 1 mg per kilogram. For other fish the
limit is 0.5 mg/kg. This directive has been incorporated in the
Netherlands in the Commodity Act. The Commodity Act also set
maximum levels for other heavy metals such as mercury, cadmium and
lead, as well as for substances such as PCBs and radioactive
substances. The residues provisions of the Pesticide Act contain
limits for pesticides.

European Directive 91/493/EEC and Directive 91/492/EEC

European Directive 91/493/EEC and Directive 91/492/EEC require
each Member State to draw up a programme in which fishery products
and molluscs and crustacea are checked for the presence of
environmental contaminants such as heavy metals and organic halogen
compounds. The Netherlands has complied with this requirement only
partly. The Dutch fish monitoring programme focuses on eel, which
can be seen as the most sensitive fish with regard to the
accumulation of residues.

According to Directive 96/23/EC, fish from fish farms will also
have to be monitored for the presence of a number of substances,
including veterinary drugs in the near future.

Synthetic substances

At present about 100,000 synthetic substances are made or used
in fields as diverse as the petrochemical industry, the production
of synthetic chemicals or iron and steel manufacture. The use of a
few of these, such as DDT and PCBs, has been forbidden or strictly
regulated in recent years. There are many other chemicals for which
it is still not clear whether they are harmful for humans.